Biofilms are formed of various layers of bacteria or micro-organisms, contained in a solid matrix. They develop to form microbian communities, one of the properties of which consists in adhering to submerged surfaces. Such adhesion is either not specific (adherence), or specific (adhesion proper) (Costerlon et al. Bacterial Biofilms, Sciences 1999; 284-6):                Adherence or reversible adhesion: Existing micro-organisms get closer to the surfaces by gravitation, Brownian motions or chimiotactism if they have flagella. During this first step of fixation which calls for only two purely physical phenomena and weak physicochemical interactions, micro-organisms can still be easily detached.        Adhesion: This slower step calls for stronger interactions as well as the microbian metabolism and the cellular appendices of the micro-organism (flagella, pilis, . . . ). The adhesion is an active and specific phenomenon. Some first pioneers are going to fix in an irreversible way to the surface more particularly thanks to the synthesis of exopolysaccharids (EPS). This process is relatively slow and depends on the present environmental factors and micro-organisms.        
Such biofilms can be found everywhere in numerous fields, wherein they entail health hazards and may cause relatively important damages.
As regards the health of human beings, for example, biofilms are responsible for infections which are more and more difficult to contain: on the whole nose-ear-throat area (auditory canal, mucous membrane of nose and sinus, eye conjunctiva . . . ), on teeth (occurrence of tartar, decays, . . . ), on bronchus, lungs (in patients suffering from mucoviscidosis . . . ), at the level of the urinary genital tract etc ( . . . ). In addition, they are the origin of most nosocomial pathologies (over 10,000 diseases per year) by forming at the level of catheters or implants (heart valves, artificial hips, urinary catheters . . . ) (J. W. Costerton, P. Stewart and E. P. Greenberg, Bacterial Biofilms: “A common cause of persistent infections”. Science, Vol. 284, pages 1318-1322).
Biofilms also concern the farm and food industry as regards their implication in food poisonings (formation upon the breaking of the cold chain, development on cutting tools, grinding tools, or on workbenches). They are also present in cooling towers and responsible for infections by the Legionelles.
The development and behavior of such biofilms remain poorly known, however, because studying them is complex, although many methods for studying the development of biofilms have been implemented.
Among the methods implemented to study the behavior of such microbian communities, the one described in WO 2005/090944 is known. That method is based on modelling the development of biofilms in a non-homogeneous, cloudy and opaque medium corresponding to the culture medium wherein micro-organisms develop to form such biofilms.
Such modelling is carried out from the measurement of the viscosity of the culture medium. The notion of viscosity only partially describes the effect of the biofilm. The biofilm is composed of:                on the one hand, a certain quantity of exopolysaccharids (EPS), or any other viscous substance, produced by micro-organisms, and        on the other hand, through a network, a meshing, of fibers and cellular bodies. Micro-organisms use cellular appendices from the micro-organism (flagella, pilis . . . ) to adhere onto the surfaces.        
The measure more specifically corresponds to a measure of viscosity and a measure of resistance to traction on cellular appendices. The measurement uses magnetizable, magnetic or electrically loaded particles (or balls) (which can be magnetized or electrically loaded under the effect of a magnetic, electromagnetic or electric field), or covered with at least one magnetic or magnetizable layer. In the following text, the term “magnetic” refers to the expression “electrically loaded” or to the terms “magnetic” or “magnetizable” or to the expression “covered with at least one magnetic or magnetizable layer,” indifferently. Such particles which exist on the surface where the biofilm is going to develop will be trapped by the viscous substance delivered by the microorganisms and by the cellular appendices used by the micro-organisms. The particles are immobilized by the two factors, in variable proportions/ratios depending on the studied micro-organisms.
Thus, the method described in the above-mentioned application consists in:                immerging at least one magnetic particle into a culture medium wherein the culture medium is preferably positioned in one or several well(s) of a micro-plate,        submitting the culture medium to a magnetic, electric or electromagnetic field, so that the particle is moved,        detecting the degree of motion freedom of the particle in the culture.        
The degree of mobility of particles is reduced or null if the viscosity increases, further to the production of EPS by the micro-organisms, or if the micro-organisms develop cellular appendices (flagella, pilis . . . ) to adhere to a surface, thus trapping the particles at the same time.
This last step c) is preferably carried out by means of an optical measurement. It deals with the lighting of the base of the wells in the micro-plate using a light source, so as to light the magnetic particle(s), and thus to determine the motion of the particle(s) in the culture medium, by comparing images. Such comparison is carried out, on the one hand, prior to and after the effect of a magnetic, electric or electromagnetic field, and on the other hand, at given time intervals (a time for micro-organism to develop and form a biofilm or not).
The optical detection devices used are conventional imaging devices (scanner, still camera or film camera). The views correspond to the base of the well, as seen from under, by transparency. The conventional optical path successively goes through the following elements:                external base of the well,        materials composing the base of the well (transparent: plastic, glass . . . ),        internal base of the well,        culture medium,        culture medium meniscus,        air above the culture medium.        
Whatever the imaging system, the experiment operator faces a major problem related to the formation of reflections between the base of the well and the liquid surface of the culture medium because of the existence of a meniscus in the culture medium. The image obtained by the optical detection device is thus particularly complex, or even sometimes impossible to process.
In addition to the difficulties in analyzing the image relating to the meniscus formed on the surface of the culture medium, the experiment operator also has to face a problem relating to the parallax between the image of the wells disposed at the center of the micro-plate, and the image of the wells disposed at the edge of the micro-plate, when he/she wishes to read simultaneously the images corresponding to each well of the micro-plate. If the base at the central well can easily be distinguished, this is not true for the base of the well at the periphery of the micro-plate. The image obtained is in fact altered by the image of the wall of the well (lower face and upper face), and the shift between the base of the well, the surface of the meniscus formed by the liquid contained in the well, and the upper opening of the well. Such air-plastic (or glass or any other transparent material), plastic (or glass or any other transparent material), culture medium, culture medium-air dioptres, create a composite image which is all the more complex as the parallax is important, as the source of reflections, more particularly, on the wall of the well.